I agree with your first coach and recommend ab rep ranges of 10-100 per set.
The rep ranges are higher on the ab exercises on purpose. Heavy weights build muscle. That shouldn't be a surprise to anyone. That is what we want for most body parts like your arms, chest, back etc.
However we don't want to enlarge the abdominals and build a large and blocky waist. The goal with abdominal training is to tighten, tone and sculpt the midsection. We aren't looking to make the abdominals larger and bulky but rather shape and sculpt them.
Does that make sense?